What are the advantages and disadvantages of constant load and constant stress creep tests?

1 Constant load creep test

A constant load creep test is performed by applying a fixed load to a specimen and measuring the change in length or strain over time. The load is usually applied by a lever arm or a dead weight, and the strain is recorded by an extensometer or a dial gauge. The advantage of this method is that it is simple and easy to set up and operate. The disadvantage is that the stress on the specimen varies with the change in cross-sectional area due to creep deformation. This means that the test results may not reflect the true creep behavior of the material under constant stress conditions.

2 Constant stress creep test

A constant stress creep test is performed by applying a fixed stress to a specimen and measuring the change in length or strain over time. The stress is usually applied by a hydraulic or pneumatic system that adjusts the load to maintain a constant force per unit area on the specimen. The strain is recorded by an extensometer or a dial gauge. The advantage of this method is that it mimics the actual service conditions of the material under constant stress. The disadvantage is that it requires a more complex and sophisticated equipment and control system, and it may be affected by factors such as temperature fluctuations, friction, and leakage.

3 Comparison of creep test types

The choice of creep test type depends on the purpose and scope of the test, the material properties, and the available resources. Constant load creep tests are more suitable for screening and ranking materials, evaluating the effects of load level and temperature on creep, and determining the minimum creep rate and the rupture time. Constant stress creep tests are more suitable for characterizing the creep behavior of materials under service conditions, evaluating the effects of stress level and temperature on creep, and determining the creep strain and the rupture strain.

4 Creep test data analysis

The data obtained from creep tests can be analyzed using various methods, such as plotting the creep curves, calculating the creep parameters, and fitting the creep models. The creep curves show the relationship between strain and time for a given load or stress and temperature. They can be used to identify the different stages of creep: primary, secondary, and tertiary. The creep parameters, such as the minimum creep rate, the rupture time, the rupture strain, and the creep modulus, can be calculated from the creep curves using empirical equations or graphical methods. They can be used to compare and classify materials, and to predict their creep performance. The creep models are mathematical expressions that describe the creep behavior of materials based on physical or empirical assumptions. They can be used to fit the creep data and to extrapolate the creep behavior beyond the test conditions.